Conventional techniques molding or forming polymer to produce a desired part involve mold temperatures consistently at or below the ejection temperature of the polymer. It has become increasingly desirable to cycle the mold to a higher temperature to improve physical properties or cosmetics for the desired part. This is done by conducting heat to the outer surfaces of the mold by an internallexternal source of heat. This process requires that surface temperatures of the polymer part exceed ejection temperatures of the part. Heating and cooling the mold lengthens the cycle time. Faster heating and cooling cycle times are required for production processes. When higher power densities are applied in response, then larger thermal gradients, which create hot and cold spots on the mold surface, negatively impact the surface appearance and cycle time. Uniform heating is critical to part quality and cycle time.
Induction heating is one approach that has been used to achieve faster heating of the mold. Induction heating occurs by exposing a work piece that is at least partially magnetic to an oscillating magnetic field. The magnetic field is typically produced by passing an alternating current through a conducting coil situated near the work piece. The applied field induces electrical eddy currents in the work piece, and the eddy currents generate heat by resistive effects. Previous methods of forming polymers have employed inductive heating using conductive polymers that include magnetic reinforcements, also known as susceptors, dispersed within the polymer matrix. The induction heating coils heat the conductive polymer matrix disposed between two non-magnetic mold surfaces. A problem with the use of magnetic reinforcements is cost, overheating during processing and adhesion of the parts to the mold cavity.
Other examples of induction heating have employed induction heating elements which are wrapped around a mold body. The mold body is non-magnetic but the mold inserts to which the polymer work piece conforms are magnetic and are heated by the induction heating elements to form the polymer. A problem, however, is that this method can be energy-intensive and it can be difficult to control thermal gradients resulting in “hot spots” along the mold surfaces. Another example of a method of induction heating includes an induction coil that is inserted between the mold halves and which heats the mold while the mold is open and then is retracted before closing the mold to produce the part. A problem is the rate of cooling, as it is very difficult to retract the coil before the mold surface cools below the targeted processing temperature.
Therefore, it is often desirable that polymers, including composite materials, replace metal parts in structural components, such as for example metal body panels in automobiles, to reduce weight and improve fuel efficiency while also meeting demanding structural requirements. Current methods of processing polymers, for example, hand lay-up, vacuum bagging, autoclaving, etc., using reinforced and unreinforced polymers, result in cycle times which are prohibitive and are not being used in higher part volume applications. Additionally, current methods typically use low-pressure processes which often result in surface finishes which are often inconsistent and require significant rework to make a usable part. As a result, current methods do not lend themselves to a high-volume production environment.